The present invention relates to the field of testing apparatus for vehicle engines and parts, and, more particularly, to measurement of spark plug gap spacing. The proper spark plug gap spacing is important to the operating of the engine. It is essential to have an accurate and convenient to operate measuring device.
A number of prior devices have been used. One in particular known to this inventor was an arrangement that incorporated the spark plug gap in the circuit of a relaxation oscillator. A DC voltage was applied across the spark plug electrodes. The gap spacing acted as a capacitor, determining the relaxation oscillator frequency. The frequency was measured and that calibrated with gap spacing. This approach had its advantages and worked for many applications. However, it did not reliably indicate gap spacing when the spark plug was in the engine and the measurement was made by connecting that test device to the engine ignition system through the distributor cap contacts. The high resistance and other, added circuit parameters prevented this type of device from operating satisfactorily.
The general approach used in the present invention is to apply an AC voltage across the spark plug gap. This approach is not new, as shown by the references listed below. Various devices have used an AC voltage or a voltage pulse. Some of these references illustrate the generally known voltage and current relation occuring when an AC voltage is applied across a spark plug, where the voltage increases until the plug fires, and then drops, and the current through a normal plug is zero until the plug fires. It is known that the voltage firing is a function of the spark plug gap spacing and it can be used to measure the gap spacing.
These and other features are shown in the references found in the preliminary novelty search conducted by applicant's attorney. All the references reported from this search are listed below:
U.s. pat. No. 2118137, issued May 24, 1938 PA1 U.s. pat. No. 2324458, issued July 13, 1943 PA1 U.s. pat. No. 2430069, issued Nov. 4, 1947 PA1 U.s. pat. No. 2649567, issued Aug. 18, 1953 PA1 U.s. pat. No. 3035224, issued May 15, 1962 PA1 U.s. pat. No. 3175149, issued Mar. 23, 1965 PA1 U.s. pat. No. 3,551,800, issued Dec. 29, 1970 PA1 U.s. pat. No. 3603872, issued Sept. 7, 1971 PA1 U.s. pat. No. 3740642, issued June 19, 1973 PA1 Ibm technical Bulletin, Vol, 13, No. 10 (March 1971) Pages 3185 to 3188, "Method for Ignition System Testing Servicing Decisions", by L.S. Briney, et al.